This application claims the priority of Korean Patent Application No. 10-2004-0055462, filed on Jul. 16, 2004, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a current mode differential transmission method and system for differentially transmitting three units of data using four signal lines.
2. Description of the Related Art
As well-known to those in the art, a signal transmission system is divided into a single ended transmission system and a differential transmission system. The single ended transmission system transmits a signal using one signal line and a common ground line. In the single ended transmission system, a base power source (for example, 0 voltage) is conducted to the common ground line, and a voltage, i.e., a potential difference, between the one signal line and the common ground line varies according to a data value to be transmitted. When the single ended transmission system transmits a digital signal, a reference voltage is necessary to determine whether a signal received in a receiver is 0 or 1. That is, the signal is determined as 1 when a received signal voltage is larger than the reference voltage and 0 when the received signal voltage is smaller than the reference voltage.
A single ended transmission method using the reference voltage has two problems when transmitting data at a high data rate. First, noise is mixed in the reference voltage, and a noise margin is reduced. Accordingly, errors are frequently generated when restoring the data. Second, a waveform of a transmitted signal is distorted due to switching noise such as ground bounce due to the high data rate transmission. Since this common mode noise cannot be removed in a receiver, errors are frequently generated when restoring the data.
Therefore, in order to transmit data at a high data rate, a differential transmission method is typically used. In the differential transmission method, a common ground line, a data line, and a complementary data line are used. That is, when data is transmitted, a signal corresponding to the data and a signal complemented to the data are transmitted via respective signal lines. In a receiver, 0 or 1 is determined according to which signal line of the two signal lines has a larger voltage value. Therefore, in the differential transmission method, since voltages of the two signal lines are used as reference voltages to each other, an additional reference voltage is not necessary. Also, since a differential mode voltage of the two signal lines is used in the receiver, the common mode noise generated due to the high data rate transmission is reduced. Accordingly, a signal transmission rate can be largely increased in the differential transmission method compared with the single ended transmission method. Besides the two advantages in the receiver using the differential transmission method, since a constant current flows on a ground pin of a chip in a transmitter driving circuit using the differential transmission method regardless of a data value to be transmitted, the switching noise is reduced. With these advantage described above, the differential transmission method is extensively used for the high data rate transmission. However, since two chip pins and two signal lines are required for transmitting one unit of data, if the number of units of data to be transmitted in a parallel way increase, the number of signal lines and the number of chip pins increase dramatically. Accordingly, a unit price of chip increases, and distribution of signal lines is complicated.